Define and use the UIHandle and TrackPanelCell protocols, but...

...no actions reimplemented to them yet.

Later commits will move special cases one at a time from TrackPanel, preserving
all click and drag capabilities at each step.  With a few exceptions, but those
lost abilities are restored in yet later commits.  (Ctrl+Click on the Label
track being one.)

src/Track.cpp

@@ -17,12 +17,13 @@ and TimeTrack.
 
 #include <algorithm>
 #include <numeric>
+#include "Track.h"
+
 #include <float.h>
 #include <wx/file.h>
 #include <wx/textfile.h>
 #include <wx/log.h>
 
-#include "Track.h"
 #include "TimeTrack.h"
 #include "WaveTrack.h"
 #include "NoteTrack.h"
@@ -318,6 +319,11 @@ void Track::SyncLockAdjust(double oldT1, double newT1)
    }
 }
 
+Track *Track::FindTrack()
+{
+   return this;
+}
+
 void PlayableTrack::Init( const PlayableTrack &orig )
 {
    mMute = orig.mMute;